Well, most of my mp3s too are of 128. Its hard to make out the difference between 128 and higher unless you have a good audio setup in your car. If you have a sub, tweets and good speakers, then it has to be atleast 256 for best output.
Here is a link with results of listening tests carried out at various bit rates.
http://www.mp3-tech.org/tests/gb/index.html